1. Field of the Invention
The present invention relates to a method for producing an X-ray lithographic mask and, in particular, to a method for producing a deep-etch X-ray lithographic mask having extremely thick absorber structures for the X-ray lithographic production of microstructures having extremely small lateral dimensions in the micron range and thicknesses of up to several hundred microns, i.e., deep-etch X-ray lithography.
2. Background of the Art
Conventional thin film masks do not produce sufficient contrast for use in deep-etch X-ray lithographic techniques where thick X-ray resist materials having a thickness of up to several hundred microns are employed and require high energy, X-ray radiation having a correspondingly great penetration depth. The absorber structures of thin film masks have a corrspondingly small thickness and are therefore too weakly absorptive to absorb the high energy, X-ray radiation. Lacking sufficient absorption contrast between carrier membrane and absorber structures, the conventional thin film masks are too transparent to X-ray radiation and the resist material sought to be exposed to radiation in a patterned image receives an unacceptable level of irradiation over its entire surface and development is impaired.
German Laid-Open patent application No. 3,338,717 discloses the production of a mask having metal absorber structures which are about 9 to 12 microns thick and are thus suitable for deep etch X-ray lithography. The mask is produced by recopying the pattern of a thin-film mask by means of soft X-rays onto a positive X-ray resist layer provided on a carrier membrane and having a thickness of about 10 to 12 microns, followed by the subsequent steps of developing to provide a resist pattern and to expose portions of the carrier membrane, and electrodepositing metal absorber structures onto the exposed portions of the carrier membrane. This mask has an absorber structure layer thickness which is within the same order of magnitude as that of the positive X-ray resist layer and is about 9 to 12 microns. During the recopying, the thin-film mask and the X-ray resist layer, in the form of a PMMA foil, that is, a metal foil which has been provided with a polymethylmethacrylate layer, are spatially and thus thermally decoupled. Thermal decoupling results in the development of temperature differences between the thin-film mask and the X-ray resist layer during irradiation which may lead to dimensional deviations and thus to transfer errors, particularly when irradiation is performed using an intensive source. Further, manipulation of such a relatively sensitive thin-film mask is difficult.
European Pat. No. 0,112,509 A3 discloses, in connection with the manufacture of a thin-film mask, the placement of an absorber structure having a thickness of about 0.25 micron directly onto a carrier membrane and the production of a second absorber structure, which is a mirror image of the first absorber structure, on the other side of the membrane by X-ray lithography using a photonegative lacquer. However, due to the limited resolution capability characteristic of photonegative lacquers, the second absorber structure has a layer thickness which is limited to about 0.25 micron. The production of masks including absorber structures having a thickness ranging from 9 to 12 microns and up, as required for the above-mentioned deep-etch X-ray lithography, is not disclosed in the reference.